Heretofore, there has been known light-emitting and light-receiving devices with a multilayered structure having a base layer made of group-III nitride semiconductor based on GaN or AlGaN with a relatively low molar fraction of AlN (see, for example, the after-mentioned Patent Publication 1 or Non-Patent Publication 1). FIG. 8 shows the structure of crystal layers in a typical conventional GaN-based light-emitting diode. This light-emitting diode includes n-GaN layer 103, an n-GaN first cladding layer 104, a GaInN/GaN multi-quantum-well active layer 105, a p-AlGaN cap layer 106, a p-GaN second cladding layer 107 and a p-GaN contact layer 108, which are deposited in this order on a sapphire substrate 101 through a low-temperature buffer layer 102 made of GaN or AIN.
For example, the GaInN/GaN multi-quantum-well active layer 105 is formed by sandwiching a GaInN quantum-well layer with 3 nm thickness between GaN barrier layers each with 10 nm thickness. After crystal growth, an ohmic semitransparent electrode 109 and a bonding pad electrode 110 each formed of a metal thin film are formed on the surface of the p-GaN contact layer 108, and an n-type electrode 111 is formed on a portion of the surface of the n-GaN first cladding layer 104 which is exposed outside through an etch process. The molar fraction of AlN in the GaInN quantum-well layer can be selectively changed to produce various light-emitting diodes having a wavelength in the range of about 380 to 620 nm.
While a semiconductor laser can be produced in a similar structure, a wavelength range capable of generating laser oscillation at room temperature is narrower than that in the light-emitting diodes.
It is known that the nitride-semiconductor crystal layers produced in the above manner include threading dislocations at a density of 108 cm−2 or more, which act as non-radiative centers. Thus, the high threading dislocation density causes deterioration in the external quantum efficiency of the light-emitting diode, and increase in the threshold current or deterioration in the element lifetime of the semiconductor laser. It is also known that threading dislocations in a photodetector, such as photodiodes, cause increase in dark current. Thus, in photodetectors, it is also regarded as one essential challenge to achieve a reduced threading dislocation density.
In late years, an epitaxial lateral overgrowth (ELO) method has been increasingly used as one technique for obtaining a reduced threading dislocation density. FIG. 9 shows the structure of a low-dislocation GaN substrate produced using the ELO method. In FIG. 9, a GaN layer 203 is grown on a sapphire substrate 201 through a low-temperature buffer layer 202 made of GaN or AlN. Equally-spaced stripe masks 204 made, for example, of SiO2 is formed on the surface of the GaN layer 203, and then a GaN overgrowth layer 205 is grown on the surfaces of the GaN layer 203 and the stripe masks 204. The crystal growth of the GaN overgrowth layer 205 is initiated only in portions of the surface of the GaN layer 203 which are not covered by the stripe masks 204, or in exposed portions of the surface of the GaN layer 203, and then crystals grow in the lateral direction to cover over the surfaces of the stripe masks 204 in a while. Finally, the GaN overgrowth layer 205 is formed as a film having a flat surface as shown in FIG. 9.
In the above crystal growth process of the GaN overgrowth layer 205, dislocations 206 to be essentially threaded perpendicular to the crystal growth direction almost never exist above the stripe masks 204 except for crystal junction areas 207. Thus, in the GaN overgrowth layer 205, areas having an extremely low threading dislocation density of about 105 to 107 cm−2 are formed above the stripe masks 204 except for the areas between the adjacent the stripe masks 204. This substrate can be used to produce a light-emitting diode or semiconductor laser reduced in non-radiative recombination centers to provide high efficiency and excellent characteristics. A photodetector produced by forming photodetector elements on the low dislocation area of the substrate can have low dark current reduced by several digits.
Lately, an AlGaN-based ultraviolet light-emitting diode grown on a bulk GaN substrate has been reported (see the after-mentioned Non-Patent Publication 2). A 305 nm ultraviolet light-emitting diode using AlInGaN multi-quantum-wells has also been reported (see the after-mentioned Non-Patent Publication 3).
[Patent Publication 1]
Japanese Patent Laid-Open Publication No. 2001-44497
[Non-Patent Publication 1]
Hiroshi Amano, et al., “Low-temperature Deposited Layer in Group-III Nitride Semiconductor Growth on Sapphire Substrate”, Journal of Surface Science Society of Japan, 2000, Vol. 21, No. 3, pp 126-133
[Non-Patent Publication 2]
Toshio Nishida, et al., “Efficient and high-power AlGaN-based ultraviolet light-emitting diode grown on bulk GaN”, APPLIED PHYSICS LETTERS, American Institute of Physics, 6 AUG. 2001, Vol. 79, No. 6, pp 711-712 [Non-Patent Publication 2]
Muhammad Asif KHAN, et al., “Stripe Geometry Ultraviolet Light-Emitting Diodes at 305 Nanometers Using Quaternary AlInGaN Multiple Quantum Wells”, The Japan Society of Applied Physics, Jpn, J. Appl. Phys., 1 Dec. 2001, Vol. 40, Part 2, No. 12A, pp. L1308-L1310